Fracture of Ductile Pipelines Subjected to Bending and Reverse Bending Loads

Abstract

Closed-form solutions for the elastic-plastic deformation and fracture responses of an offshore pipeline during reeling and unreeling operations are derived. As an example, an API 5L X52 pipeline of thickness 25.4 mm (1 in.) and outer diameter 406 mm (16 in.) bent over a drum radius of 16.5 m (54 ft) and then straightened back to its original configuration is considered. It was found that the pipeline underwent significant plastic deformation during bending and reverse bending. The plastic zone extended from the top (bottom) of the pipe to 16.46 mm above (below) the neutral axis during bending. Modified J-integral and JR resistance curves were compared to evaluate through-thickness crack growth and stability. The difference between predicted crack growth and experimental results varied from 30 to 140 percent. J-integral-based crack growth predictions were significantly lower and much closer to experimental results than previous predictions based on crack tip opening displacement. Finally, a failure assessment diagram (FAD) for the pipeline was presented. The FAD would enable one to assess the propensity to fracture in pipelines.